For many machining and woodworking operations it is necessary to secure a workpiece to a machine bed or work surface. It is standard when the bed is formed of steel to provide it with upwardly open grooves of inverted T-section into which is fitted a T-nut or T-bolt that secures the clamp in place.
Such a clamp typically comprises a post having a lower end that is secured to the work surface. A body is displaceable vertically and axially along this post and normally has an arm whose free end can be brought to bear on the tool or workpiece being clamped to the surface to press it down against the work surface.
In U.S. Pat. No. 4,238,124 of Eichfeld the body is formed as a split ring which can be clamped by circumferential tightening at any angular or axial position on the post. Thus the position, both angular and axial, can be adjusted steplessly. Such a system does not make it possible to accurately duplicate a given angular position which is needed in many setups.
German patent 3,333,813 of Schmid discloses a double-jaw work-surface clamp. A collet arrangement is used to control the height or axial position of the upper jaw, and the lower jaw is movable axially by a bolt extending between the jaws. This system is also steplessly adjustable with no way to reproduce a given angular position.
German patent document 4,321,387 of Wurche describes a positioner having an upright post on which is mounted a ring carrying a radially projecting arm. The post has a single radially outwardly open and axially extending groove in which engages a spring-loaded ball mounted in the spring so as to define one standard angular position for the arm. Such a device is not a hold-down clamp; instead it serves merely for accurately setting the position of a workpiece that is then secured in place by standard hold-down clamps. The vertical or axial position of the arm-carrying ring is changed by moving a pair of abutment rings that axially closely flank it and that are each secured in place by a respective set screw, so that adjusting the vertical position is a laborious operation.
German utility model 297 03 707 assigned to HMC-Brauer Ltd. describes a hold-down clamp having a body that is vertically displaceable to a limited significant extent on the post, and whose free end is provided with an extensible bolt acting as a hold-down foot. The angular position of the arm with respect to the post is fixed by an interfitting axially extending groove and rib respectively formed on the body and post. A tightening lever can vertically displace the body through a short stroke so that, once the device is set the clamping can be effected with a single turn of this lever. This device must be carefully adjusted for each setup and, for a new setup, all the adjustments must be changed. There is no simple way to change the angular orientation of the arm relative to the post.